Mixer diodes are widely employed in the microwave receivers for mixing of a local oscillator signal with the received signal. Such diodes operate with very low incoming signal levels as there is ordinarily no amplifying stage ahead of the diodes. The low level of the received signals necessitates an extremely small junction area for the diodes. In the prior art, diodes for the above stated mixing purpose were fabricated by covering a body of semiconductor material with a layer of quartz, drilling an extremely small hole through the quartz and into the semiconductor material and plating a Schottky barrier junction through the hole and onto the surface of the body of semiconductor material exposed through the hole extending onto the quartz. Unfortunately, diodes fabricated with this process have been found to have undesirably high values of parasitic capacitance between the metal forming the Schottky barrier layer and the body of semiconductor material. Such high capacitance reduced the efficiency of the diodes as well as the maximum frequency at which such diodes could be profitably employed. The high capacitance arose primarily from two factors. First, the practical upper limit for the thickness of the quartz layer has been restricted by present semiconductor processing techniques to approximately one micron. Secondly, quartz has a relatively high dielectric constant compared to the dielectric constant of low conductivity depleted semiconductor material.
Moreover, prior art devices suffered from low reverse bias breakdown voltage since the abrupt edges of the Schottky barrier metal layer lay directly upon the surface of the semiconductor layer with which the junction was formed.